Summary of Work: The long-term goal of this project is to understand the fidelity of DNA synthesis by multiprotein DNA replication and repair complexes. This year, progress was made in four areas. We established the fidelity of the mitochondrial replicative DNA polymerase gamma with and without exonucleolytic proofreading and with and without the p55 processivity subunit. We obtained evidence suggesting that errors made by an active site mutant of DNA polymerase gamma contribute to a human hereditary disease, Progressive External Ophthalmoplegia. We established an in vivo system in yeast cells to determine the fidelity of DNA replication by the leading and lagging strand DNA replication machinery and used the sysyem to demonstrate that yeast origins establish a strand bias for replicational mutagenesis. We compared the error specificity of human DNA polymerase eta during copying of an immunoglobulin transgene to the base substitution specificity of somatic hypermutation. The results suggest that DNA polymerase eta contributes to the somatic hypermutagenesis required for development of a diverse population of mature high affinity antibodies in normal humans. Generally, such studies of how genomes are efficiently and correctly replicated and repaired are important for human health because spontaneous and DNA damage-induced replication errors are likely sources of mutations that may initiate human diseases, while at the same time, specialized regulated mutagenesis is required for normal development.